The present disclosure generally relates to a polymeric cellular confinement system which can be filled with soil, concrete, aggregate, earth materials, and the like. More specifically, the present disclosure concerns a cellular confinement system characterized by improved durability against damage generated by ultraviolet light, humidity, aggressive soils, and combinations thereof.
Plastic soil reinforcing articles, especially cellular confinement systems (CCSs), are used to increase the load bearing capacity, stability and erosion resistance of geotechnical materials such as soil, rock, sand, stone, peat, clay, concrete, aggregate and earth materials which are supported by said CCSs.
CCSs comprise a plurality of high density polyethylene (HDPE) strips in a characteristic honeycomb-like three-dimensional structure. The strips are welded to each other at discrete locations to achieve this structure. Geotechnical materials can be reinforced and stabilized within or by CCSs. The geotechnical material that is stabilized and reinforced by the said CCS is referred to hereinafter as geotechnical reinforced material (GRM). The surfaces of the CCS may be embossed to increase friction with the GRM and decrease relative movement between the CCS and the GRM.
The CCS strengthens the GRM by increasing its shear strength and stiffness as a result of the hoop strength of the cell walls, the passive resistance of adjacent cells, and friction between the CCS and GRM. Under load, the CCS generates powerful lateral confinement forces and soil-cell wall friction. These mechanisms create a bridging structure with high flexural strength and stiffness. The bridging action improves the long-term load-deformation performance of common granular fill materials and allows dramatic reductions of up to 50% in the thickness and weight of structural support elements. CCSs may be used in load support applications such as road base stabilization, intermodal yards, under railroad tracks to stabilize track ballast, retaining walls, to protect GRM or vegetation, and on slopes and channels.
The term “HDPE” refers hereinafter to a polyethylene characterized by density of greater than 0.940 g/cm3. The term medium density polyethylene (MDPE) refers to a polyethylene characterized by density of greater than 0.925 g/cm3 to 0.940 g/cm3. The term linear low density polyethylene (LLDPE) refers to a polyethylene characterized by density of 0.91 to 0.925 g/cm3.
The plastic walls of the CCSs may become damaged during service and use in the field by UV light, heat, and humidity (UHH). The damage results in brittleness, decreased flexibility, toughness, impact and puncture resistance, poor tear resistance, and discoloration. In particular, heat damage to the CCS is significant in hot areas on the globe. As used herein, the term “hot areas” refers to areas located 42 degrees latitude on either side of the equator and especially along the desert belt. Hot areas include, for example, North Africa, southern Spain, the Middle East, Arizona, Texas, Louisiana, Florida, Central America, Brazil, most of India, southern China, Australia, and part of Japan. Hot areas regularly experience temperatures above 35° C. and intensive sunlight for periods of up to 14 hours each day. Dark surfaces of plastics exposed to direct sunlight can reach temperatures as high as +90° C.
Some strategies have been applied industrially in order to protect the plastic walls from this damage by treating the polymer making up the plastic walls. For dark colored products, e.g., black or dark gray products, carbon black can be introduced to block UV light and dissipate free radicals. However, one disadvantage produced through the use of carbon black is its aesthetic appearance. Black CCSs are less attractive in applications where the CCS is part of a landscape structure. A second disadvantage is that black CCSs tend to absorb sunlight and heat up. HDPE and MDPE tend to creep when heated above 40-50° C. As a consequence, creep can be severely accelerated, especially in the welding points and thinner wall structures, potentially resulting in structural failures.
CCSs are usually immobilized or anchored to the GRM by wedges, tendons, bars, or anchors. This immobilization is especially crucial when the CCS is used to reinforce a slope. The wedges, tendons, bars, or anchors are usually made of iron, and can be heated by direct sunlight to temperatures that may exceed 60-85° C. The high conductivity of iron also transmits the heat to the buried portion of the CCS. These anchor points are subjected to severe stress concentrations. Without UHH protection, these anchor points may fail before any significant damage is observed in the rest of the CCS.
Stress is also generated at the welds between the strips making up the CCS. Stress can be applied from compression when humans walk over the CCS during installation, before and while it is filled with GRM, or when GRM is dumped onto the CCS to fill the cells. GRM can also expand when it becomes wet or when water already in the GRM freezes in cold weather. In addition, GRM has a coefficient of thermal expansion (CTE) about 5-10 times lower than the HDPE used to make the strips. Thus, the HDPE will expand much more than the GRM; this causes stress along the CCS walls and especially at the welds.
Some CCSs are pigmented to shades similar to the GRM they support. These include light colored products and custom-shaded CCSs, such as soil-like colored CCSs, grass-like colored CCSs and peat-like colored CCSs.
For CCSs, special additives (i.e. other than carbon black) are required in order to maintain their properties for periods of 20 years or more. The most effective additives are UV absorbers such as benzotriazoles and benzophenones, radical scavengers such as hindered amine light stabilizers (HALS), and antioxidants. Usually, “packages” of more than one additive are provided to the polymer. The additives are introduced into the polymer, usually as a master batch or holkobatch, a dispersion, and/or solution of the additives in a polymer carrier or a wax carrier.
The amount of additives in the polymer used to make the CCS depends on the life-time required for the CCS. To provide protection for periods of about 5 years, the amount of additives needed is less than if protection for a period of 10 years or more is required. Because additives leach out of the polymer, evaporate, or hydrolyze over time, the actual amount of additives required for protection over a long period of time is about 2 to 10 times greater than the amount that is needed for short term protection needs. In other words, the amount of additives added to the polymer must compensation for leaching, evaporation, and hydrolysis and is thus significantly greater than amount needed for short term protection. Moreover, as the heat and humidity where the CCS is to be used increases, more additives need to be added to the polymer to maintain its protection level.
The additives are generally dispersed or otherwise dissolved fairly evenly throughout the entire cross-section of the polymeric strips used to make the CCS. However, most interaction between the additives and the UHH damage-causing agents takes place in the outermost volume, i.e. 10 to 200 microns, of the polymeric strip or film.
Some hot areas, especially tropical areas, also experience high humidity and heavy rains. The combination of high humidity and heat accelerates the hydrolysis, extraction and evaporation of the protective additives from the polymeric strip. The most significant is the loss of UV absorbers, such as benzophenones and benzotriazoles, and heat stabilizers—especially hindered amine light stabilizers (HALS). Once such additives are lost, the polymeric strip is easily attacked and its properties deteriorate rapidly.
U.S. Pat. No. 6,953,828 discloses a membrane, including a geomembrane, stabilized against UV. The patent relates to polypropylene and very low density polyethylene compositions that are effective as membranes, but are not practical for CCSs. Polypropylene is too brittle at sub-zero temperatures. Very low density polyethylene is too weak for use in a CCS because it tends to creep under moderate loads. Once a CCS creeps, the integrity of the CCS and GRM is disrupted and structural performance is irreversibly damaged. In addition, polypropylene requires a large loading of additives to overcome leaching and hydrolysis; this results in an uneconomical polymer.
U.S. Pat. No. 6,872,460 teaches a bi-layer polyester film structure, wherein UV absorbers and stabilizers are introduced into one or two layers. Various grades of polyesters are generally applicable for geo-grids, which are two-dimensional articles used to reinforce soil, such as a matrix of reinforcing tendons. Geo-grids are usually buried underground and thus not exposed to UV light. In contrast, CCSs are three-dimensional and are usually partially exposed above ground level, thus exposed to UV light. Polyesters are generally unsuitable for CCSs due to their stiffness, poor impact and puncture resistance at ambient and especially at sub-zero temperatures, medium to poor hydrolytic resistance (especially when in direct contact with basic media such as concrete and calcined soils), and their overall cost. Again, polyesters require a large loading of additives to overcome leaching and hydrolysis; this results in an uneconomical polymer.
For thin polymeric strips (characterized by a thickness of less than about 500 microns), the actual amount of additive required generally matches the theoretical calculated required amount. In thicker strips (characterized by thickness of more than about 750 microns—that is usually the case with structural geotechnical reinforcing elements—CCS as example), however, the actual total amount of additive required is generally much higher than the theoretical calculated required amount. For high performance CCSs having thicknesses of about 1.5 mm or more, wherein strength, toughness, flexibility, tear, puncture resistance, and low temperature retention are required, the total amount of additive required is generally 5 to 10 times higher than the theoretical calculated required amount. UHH-protecting additives are very expensive relative to the cost of the polymer. Most manufacturers therefore provide the additives at loadings more closely matching the low (i.e. minimal) theoretical calculated loading level, not the higher loadings required for long-term protection for periods of 50 years and more. Moreover, HDPE and MDPE provide poor barrier properties against ingress of harmful ions and molecules into the polymer, and against leaching and evaporation of the additives from the polymer. Because of this, in reality, most manufacturers do not currently guarantee long-term durability of their thick polymeric strips. Current CCSs use HALS and UV absorbers in the amount of 0.1 to 0.25 weight percent dispersed throughout the polymeric strip.
Another aspect related to outdoor durability is the type of polymer used for the CCS. Selection of the correct polymer for this application is a tradeoff between economy, i.e. cost of raw materials, and long-term durability. In this regard, polyethylene (PE) is one of the most popular materials for use, due its balance of cost, strength, flexibility at temperatures as low as minus 60° C., and ease of processing in standard extrusion equipment. Moreover, polyethylene is moderately resistant against UV light and heat. However, without additives, polyethylene is susceptible to degradation within one year to a degree that is unacceptable for commercial use. Even when heavily stabilized, PE is still inferior relatively to more UV-resistant polymers, such as ethylene-acrylic ester copolymers and terpolymers.
On the other hand, polymers that exhibit higher UV and heat resistance, such as acrylic and methacrylic ester copolymers and terpolymers, and specifically ethylene-acrylic ester copolymers and terpolymers, are very suitable to commercial application from the standpoint of UHH resistance. However, their relatively high cost and relatively low modulus and strength characteristics limit their wide-scale use in CCS applications.
There is a need to provide a cost-effective UHH-resistant polymeric strip and CCSs comprising the same, especially GRM-like light colored strips and CCSs thereof. Such CCSs are resistant to harsh conditions, especially outdoor applications, at climates ranging from arid, tropic, and subtropic to arctic, and have a useful service life of 50 years and more.